86 ILLINOIS STATE ACADEMY OF SCIENCE 



DISCUSSION 



In order to interpret the above data correctly it is 

 necessary to keep in mind the fact that the figures in 

 columns 2 to 7 represent in each case the number of 

 seconds required for the color change in the cobalt paper 

 to take place. A large number therefore indicates a low 

 transpiration rate while a small number indicates a high 

 transpiration rate. Likewise a plus in the difference col- 

 umns indicates a high transpiration of the weed as com- 

 pared with corn, while a minus shows that the rate in 

 the case of the weed was low as compared to corn. With 

 these facts in mind it is seen readily from the averages 

 at the end of the table that all of the weeds used have 

 higher transpiration rates, per unit area, than corn. Se- 

 taria, which is a grass and therefore closely related to 

 corn, has a transpiration rate only slightly higher than 

 corn since the average difference (column A-D) is only 

 17.6 seconds. The four dicotyledonous weeds, however, 

 all have rates considerably higher than that of corn, the. 

 average differences varying from 45 seconds in the case 

 of Abutilon to 94.8 seconds in the case of Ambrosia. 



As seen from the table, the plants used, when arranged 

 in the order of decreasing transpiration rate, stand as 

 follows : Ambrosia, Polygonum, Sida, Abutilon, Setaria 

 and corn. This order is fairly consistent with the num- 

 ber of stomata per unit area on the leaves of these plants. 

 Corn and Setaria have approximately equal numbers of 

 stomata per unit area and they are about evenly distrib- 

 uted on the upper and lower leaf surfaces. The numbers 

 are very small, however, as compared to those found on 

 any one of the dicotyledonous weeds, though the latter 

 have many more stomata on the lower than on the upper 

 surfaces. 



The work described in this paper proves that corn 

 transpires more slowly area for area than the weeds 

 with which it was compared. Whether or not this fact 

 is an economically important one under any conditions 

 has not been demonstrated. Kiesselbach (10) showed 

 conclusively that weeds do rob corn of some needed mois- 

 ture, but other factors such as food supply, light rela- 

 tions, etc., have usually been considered of more im- 



